ENVIRONMENT
THE POLLUTEC STORMWATER POLLUTION TRAP: FIELD TRIALS R A Allison, T H F Wong, T A McMahon Abstract A new pollutant trapping device, based on a solid separation mechanism referred to as continuous deflective separation, is being developed by Pollutec Pty Ltd. A 6 m x 6 m x 4 m . system was constructed alongside a 1220 mm diameter main drain in Coburg, an inner city suburb of Melbourne, then connected into the flow. A joint research project involving a number of parties aims to compare effectiveness and costs of three methods of trapping gross pollutants in the urban environment: street channel entry pits, traps within main drains , and traps installed in slow-moving receiving waters. This paper describes the installation of the Pollutec trap , early results on its performance and the characteristics of the material collected.
Keywords Stormwater, gross pollutants, pollutant traps
Introduction
based on a solid separation mechanism referred to as continuous deflective separation (CDS), and developed by Pollutec Pty Ltd, is being trialed as part of the project. Its functionality has been reported by Wong and Wootton (1995), and is briefly described below. The field component of the study "involves monitoring a CDS unit and a number of side entry pit traps (Banyule City Council) in the same catchment. The trapping performance and cleaning requirements of each system will be examined and will provide useful information for the decision-making protocol. The characteristics of each method will be compared in terms of costs and the volume and type of material collected. Previous monitoring in the Coburg area (Allison and Chiew, 1995) found that" natural material (leaves and twigs) contributes at least two thirds of gross pollutant loads and commercial areas were found to have larger amounts of human-derived materials (paper and plastics) than other areas. Storm event monitoring also showed that the highest concentrations of gross pollutants were during the early stages of runoff events. However, the largest quantities of material (loads) were being transported during times of high discharge. The first part of the field studies
concerns the CDS unit trap. This paper discusses the technology of the CDS system, the construction of the CDS unit in the Melbourne suburb of Coburg and preliminary results from the field work. Two objectives of the field work are to determine the trapping effectiveness and hydraulic performance of the CDS unit and to investigate the types and amounts of litter and debris which are transported in stormwater drains . A feature of the project is the range of parties it brings together: Melbourne Water (Parks and Waterways) , Pollutec Pty Ltd, the Cooperative Research Centre for Catchment Hydrology, Commonwealth EPA, Moreland City Council, Banyule City Council and the Merri Creek Management Committee.
Background The CDS mechanism of solid separation is accomplished by diverting the incoming flow and associated pollutants away from the main flow stream of the pipe or waterway into a pollutant separation and containment chamber. As described by Wong and Wootton (1995), solids within the separation chamber are kept in continuous motion and prevented from 'blocking' as commonly observed with direct filtration systems. This is achieved by hydraulic design which ensures that the force exerted on an object by the circular flow action is significantly higher than that caused by the pressure differential across the separation screen. Figure 1 is a schematic diagram of a typical system. It should be noted that the water, while forming a vortex in the centre of the screen chamber, is forced to reverse its flow in order to pass through the screen apertures . However, solids are swept across the screen and concentrated in the central vortex. Floating objects are kept in continuous motion on the water surface while' the heavier pollutants settle into the containment sump . The screen itself is effectively self-cleaning. As noted above, the unit
Litter and debris (gross pollutants) from urban stormwater channels are major pollutants to receiving waters. They are aesthetically unpleasant, smell, attract vermin and are a potential source of nutrients as they break-up in catchments and waterways . The CRC for Catchment Hydrology is the focus for a large project aimed at developrng a decision-making Inlet protocol so that authorities Scree n can determine effective approaches for trapping gross pollutants within a particular urban drainage area . The protocol considers three --+i----- Overf lo w l. categories of trapping location: at street channel !.\. entry pits, within main I, drains, and in slow-moving receiving waters. It takes into Outlet I Chamber I account the urban drainage system layout, the predominant land-use and funding limitations in assessing the benefits and costs of the devices. A gross pollutant trap Figure 1 Schematic representation of the CDS structure
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